The invention provides a drug which inhibits BCRP.A breast cancer resistance protein inhibitor containing, as an active ingredient, a diphenylacrylonitrile derivative represented by the following formula (1): [wherein, each of 8 R's, which are the same or different from one another, represents a hydrogen atom, a hydroxyl group, a nitro group, an amino group, an acetylamino group (—NHCOCH3 group); a cyano group (—CN group); a formyl group (—CHO group), —COOR1 (R1 is hydrogen or C1-C4 alkyl), —O(CH2)nCOOR2 (n=1−7: R2 is hydrogen or C1-C4 alkyl), —OOCCH2CH2COOR3 (R3 is hydrogen, C1-C4 alkyl, (Z)-2-(3,4-dimethoxy-phenyl)-3-(4-hydroxy-phenyl)-acrylonitrile, or glycopyranosyl), a C1-C8 alkoxy group, a C1-C4 alkyl group, a halogen atom, a C1-C4 alkoxy C1-C4 alkoxy C1-C4 alkoxy group, a C2-C8 acyloxy group, a C2-C8 halogenoacyloxy group, a methylenedioxy group, a trifluoromethyl group, a phosphate group (i.e., —OP(O) (OH)2) or a salt thereof, a sulfate group (i.e., —OSO3H) or a salt thereof, a glycopyranosyl group or a salt thereof, a phosphate ester of a glycopyranosyl group or a salt of the ester, a sulfate ester of a glycopyranosyl group or a salt of the ester, or a piperidinopiperidinocarbonyloxy group], an ester thereof, or a salt thereof.